There has been known a method for producing a carbon fiber bundle, including heat-treating a carbon-fiber-precursor acrylic fiber bundle in an oxygen atmosphere at 200° C. to 400° C. to convert it into a flameproofed fiber bundle (flameproofing step) followed by carbonizing the flameproofed fiber bundle in an inert atmosphere at 1000° C. or higher to obtain the carbon fiber bundle (carbonization step). The carbon fiber bundle obtained by this method is widely used industrially, particularly as a reinforcing fiber for composite materials because of its excellent mechanical properties.
However, in this method for producing a carbon fiber bundle, a fusion may occur between single fibers in the flameproofing step, leading to process failure, such as fluffing and bundle breakage in the flameproofing step and the subsequent carbonization step. (The flameproofing step and the carbonization step are hereinafter also referred to comprehensively as the “calcination step.”) As a method to prevent the fusion between single fibers in the flameproofing step, application of an oil composition on the surface of the carbon-fiber-precursor acrylic fiber bundle (oil composition application treatment) is known, and a large number of oil compositions have been studied.
As an oil composition, silicone-based oils containing a silicone as the main component have been generally used. As a silicone, modified silicones that have reactive groups, such as amino, epoxy, and polyether, are generally used because of their compatibility with and retention on a carbon-fiber-precursor acrylic fiber bundle.
However, upon heating, modified silicone-based oils tend to become highly viscous because of progression of crosslinking reaction, and the resulting viscous material tends to accumulate on the surfaces of the fiber conveying rollers and guides that are used in the after treatment (such as the drying step) of the carbon-fiber-precursor acrylic fiber bundle with an oil composition adhering thereto and the flameproofing step. Thus, the carbon-fiber-precursor acrylic fiber bundle with an oil composition adhering thereto or the flameproofed fiber bundle may be entwined or tangled around the fiber conveying rollers and guides, causing process failure, such as bundle breakage and leading to decreased operability.
In addition, silicone-based oils that have been applied to the carbon-fiber-precursor acrylic fiber bundle tend to produce silicon compounds, such as silicon oxides, silicon carbides, and silicon nitrides, in the calcination step. Production of silicon compounds is known to lead to a low industrial productivity and a low product quality.
Thus, oil compositions with a lower content of silicone have been proposed with the aim of reducing the silicone content of the carbon-fiber-precursor acrylic fiber bundle with an oil composition adhering thereto. For example, an oil composition with reduced silicone content has been proposed that contains 40 to 100 wt % of an emulsifying agent containing 50 to 100 wt % of a polycyclic aromatic compound (see Patent Document 1).
In addition, an oil composition with reduced silicone content has also been proposed that contains 80 to 95% by mass of a higher fatty acid ester product on both sides of an ethylene oxide and/or propylene oxide adduct of bisphenol A (see Patent Document 2). An oil composition with a reduced silicone content has also been proposed that contains a product of reaction of a saturated aliphatic dicarboxylic acid and a mono-alkyl ester of an ethylene oxide and/or propylene oxide adduct of bisphenol A (see Patent Document 3).
An oil composition has also been proposed that combines a heat-resistant resin with a residual rate of 80% by mass or more after heating in air at 250° C. for 2 hours and silicone (see Patent Document 4).
Furthermore, an oil composition has also been proposed that contains 10% by mass or more of a compound having a reactive functional group and no silicone compound or that contains a silicone compound in a quantity within the range of 2% by mass in terms of the mass of silicon (see Patent Document 5).
On the other hand, an oil composition with lower silicone content has been proposed that contains a compatibilizing agent with the aim of increasing the affinity of the silicone-based compounds and the non-silicone-based compounds towards each other to mix them (see Patent Documents 6 and 7).
An oil composition has recently been proposed that contains as essential components an ester compound having at least 3 ester groups in the molecule and a silicone-based compound (see Patent Document 8). In the oil composition, the silicone content is reduced by the use of an ester compound. In addition, both the prevention of fusion between single fibers and a stable operation can be achieved simultaneously in the production of carbon fibers.